RT Journal Article
SR Electronic
T1 Discussion
JF Quarterly Journal of the Geological Society
JO Quarterly Journal of the Geological Society
FD Geological Society of London
SP 117
OP 118
DO 10.1144/gsjgs.122.1.0117
VO 122
IS 1-4
YR 1966
UL http://jgs.lyellcollection.org/content/122/1-4/117.abstract
AB Dr S. Moorbath commented that while the ages of the Newer Granites were not entirely unexpected, it was evident that the Oughterard granite Rb–Sr whole-rock age of 510 ± 35m.y. was the most significant date yet to emerge from geochronological studies in the west of Ireland. It raised the problem of the interpretation of the generally somewhat lower isotopic mineral ages from Connemara. K–Ar and Rb–Sr age determinations on about 50 micas and hornblendes (Bell, Moorbath, Leake & McKerrow, in the press) clearly showed that a major isotopic event affected the Dalradian Connemara Series about 440 to 460 m.y. ago. There was no correlation between the type of lineation and the measured age. If one accepted (i) the circumstantially overwhelming geological evidence for a pre-Arenig age of the major folding and metamorphism of the Connemara Series, (ii) an age of 500 ± 15 m.y. for the base of the Arenig, (iii) an age of 510 ± 35m.y. for the age of intrusion of the post-F4 (possibly post-F5) Oughterard granite, then it followed that the isotopic mineral ages did not reflect pre-Arenig events and could not be correlated with the major foldings and metamorphisms. The speaker and his colleagues postulated that the isotopic mineral ages reflected either (i) uplift, cooling, and unroofing of the Connemara massif in Mid-to-Upper Ordovician times when diffusion of radiogenic isotopes ceased, and also corresponding roughly to maximum sedimentation in the Mayo trough, and/or (ii) widespread retrograde metamorphism with accompanying chloritization, possibly connected with